This invention relates to a process for the production of a heat-treatable low emissivity coated glass and to a heat-treatable low emissivity coated glass.
It is known that transparent glass substrates with a thin reflective metal coating, typically silver 5 nm to 30 nm thick, may be produced with low emissivity and a high visible light transmission i.e. which reflect a high proportion of infra-red radiation incident upon them but allow visible radiation to pass through. The use of such coatings on window glass leads to a reduction in heat loss. For optimum light transmission, the silver layers are sandwiched between thin anti-reflection layers of metal oxide. Such low emissivity coatings are produced by vacuum deposition processes, especially sputtering, and normally comprise a thin layer of silver sandwiched between two layers of metal oxide. Such coatings are described, for example, in UK patent specification GB 2 129 831.
Unfortunately, such low emissivity coatings are not stable to heating in air, and the coating loses its properties of low emissivity and high light transmission when the coated glass is subjected to a thermal cycle which cycle is required for bending or toughening the glass. To obtain a toughened or bent glass substrate bearing a low emissivity reflective metal coating that has high light transmission it has been necessary to either bend and/or toughen the glass before depositing the reflective metal coating or to deposit an additional, protective layer over the reflective metal layer before bending/toughening. GB patent 2 186 001B describes this problem and a process for the production of a bent and/or toughened silver coated glass substrate wherein an additional metal layer of aluminium, titanium, zinc or tantalum is deposited over the silver layer. The additional metal layer protects the silver layer beneath it from degradation during the bending or toughening thermal cycle by acting as a barrier layer between the silver layer and the oxidising atmosphere.